Mailing list w/ external addresses: Distribution Groups or Listserv?

I'm being asked to setup a mailing list for leaders of several similar companies which would have an email address domain different than the Zimbra server's currently hosted domain. Is it possible to have external email addresses as members of a Distribution Group in Zimbra? I noticed you can define which external addresses you want to allow to email the Distribution Group, but I have also read that a Zimbra Distribution Group won't distribute the emails to addresses outside of its domain. Is Zimbra+Mailman my only option here?

I then tried to send an email from myemail@mycomapny.com to test-list@xyz.com, but did not receive the email I sent back to myself. I added another email address of my own to the test-list@xyz.com distribution group, and my email to the distribution group wasn't sent to either of my addresses. Am I missing something, or can Zimbra Distribution Groups only send to accounts on the same domain as the zimbra server?